Typically, when a crank set is installed on a bicycle, the ball bearings of the bottom bracket assembly must be preloaded in order for the crank set to properly function. Preloading the bearings eliminates lateral movement of the crankset and applies a pressure to the bearings of the bottom bracket so that the bearings only move in a wanted direction and do not give in the radial or axial direction. Optimal preloading of the bearings eliminates side-to-side play of the crankset, which may result in a dropped chain, and also prevents the bearings from being too heavily loaded, which decreases the life of the bearings and may cause the crank to seize.
The size of the bottom bracket shell, the bottom bracket, the design of the crank set and/or the design of the crank arms influence how much preload may be required for optimal performance. Consequently, a crank set and bottom bracket assembly is often found as an integrated system including complementary sized components to adapt to the bicycle frame. Additionally, spacers such as elastomer washers or caps which are able to be tightened may be used in order to eliminate play in the crank arms and achieve a proper bearing preload. In either case, properly adjusting the preload of the bottom bracket bearings requires a variety of tools and may require readjustment as the bearings are broken in and play develops within the crank arms and the bottom bracket assembly.